1. Field of the Invention
The present invention relates to a surface-emitting device, a front light, and a liquid crystal device, and more particularly, to the structure of a surface-emitting device suitably used as a front light for a liquid crystal display device.
2. Description of Related Art
Conventionally, reflective liquid crystal display panels that do not consume large amounts of power are used in portable devices and the like, whereas the displays thereof are not visible in dark conditions, such as during the night. In contrast, since transmissive liquid crystal display panels have backlights, displays thereof are visible even in dim conditions. However, the backlights consume large amounts of power, and the displays are rather difficult to view outdoors in bright daylight.
In order to solve the above problems, a liquid crystal display device having a front light serving as a surface-emitting device has been proposed in which a light guide plate is placed in front of a reflective liquid crystal display panel, light from a light source, such as a cold cathode-ray tube, placed adjacent to the end of the light guide plate, is introduced into the light guide plate, and the light is emitted from the surface of the light guide plate toward the liquid crystal display panel, thereby allowing the display to be visible even in a dark environment. In the liquid crystal display device having a front light, since the liquid crystal display panel is visible through the light guide plate in the daytime, it can be used as a normal reflective liquid crystal display panel. In a dark environment, the liquid crystal display panel is illuminated by lighting the front light, so that the display is visible.
The above-described conventional front light has a configuration in which light is substantially uniformly introduced into the light guide plate by using a linear light source, such as a cold cathode-ray tube, so as to illuminate the liquid crystal panel. In small devices, such as portable devices, it is sometimes impossible, from the viewpoint of cost and capacity, to use the cold cathode-ray tube or the like, and the cold cathode-ray tube consumes too large amount of power.
For this reason, it may be possible to reduce the cost, size, and power consumption of the devices by using a point light source, such as a light-emitting diode, which is inexpensive and which consumes a small amount of power. In this case, however, since the light-emitting diode is a point light source and has directivity in the light emitting direction, light is not uniformly emitted toward the light guide plate. As a result, the light intensity distribution (in-plane distribution) of illumination light for the liquid crystal display panel is made nonuniform, and this impairs visibility in a dark environment.
As shown in FIG. 5, in a case in which a front light 10 is provided using a light guide plate 12 composed of convex portions, each having a triangular cross section and consisting of a gently inclined face 12a and a steeply inclined face 12b, arranged on its surface in parallel and in stripes in plan view, the front light 10 is set in most cases to be viewed from the F-direction in the figure, which is slightly inclined with respect to the normal to the surface of the light guide plate 12, in order to enhance effective display brightness during illumination. As shown in FIG. 6, when a linear light source, such as a cold cathode-ray tube, is used as a light source, multiple bright lines are viewed as transverse lines due to light leaking from the steeply inclined faces 12b. As shown in FIG. 7, if point light sources 13, such as light-emitting diodes, are used as light sources in this case, short bright lines are viewed in the form of bars along the light guiding direction from the light source, which is different from the case of the linear light source, such as a cold cathode-ray tube, and therefore, visibility is further reduced, compared with the case of the linear light source.
Accordingly, the present invention has been made to overcome the above problems, and an object of the present invention is to provide a surface-emitting device, such as a front light, in which visibility is not reduced even when using a point light source such as a light-emitting diode.
An Exemplary embodiment that the present invention provides to overcome the above problems is a surface-emitting device including a transmissive light guide plate for emitting from its surface light propagating therein in a predetermined direction along the surface, and a light source placed adjacent to an end of the light guide plate so as to introduce light into the light guide plate in the other direction that is different from the predetermined direction, wherein a reflecting layer is disposed at an end of the light guide plate different from the end where the light source is placed, so as to reflect light, which propagates inside the light guide plate in the other direction, in nearly the predetermined direction inside the light guide plate.
According to this exemplary embodiment, light introduced from the light source into the light guide plate travels inside the light guide plate, is reflected by the reflection plate, travels in nearly the predetermined direction, and is emitted from the surface of the light guide plate. Since this allows a long optical path length from the light source to the emitting position, in-plane uniformity of emitted light can be enhanced when the light source has directivity, when the light source is a point light source, or when the distribution of light introduced from the light source into the light guide plate is nonuniform. When the surface-emitting device is used as a front light placed in front of various display devices, since nonuniformity of distribution of leakage light leaking frontward can be reduced, visibility can be improved. The other direction described above may be, for example, a direction along the surface of the light guide plate and opposite from the predetermined direction.
According to the above exemplary embodiment, it is preferable to interpose a light-scattering layer between the light guide plate and the reflecting layer. When the light-scattering layer is placed before the reflecting surface, light is scattered before and after reflection, in-plane uniformity of emitted light can be further enhanced.
According to the above exemplary embodiment, the light source may have directivity in the light emitting direction inside the light guide plate, and the light source may be a point light source. Particularly in these cases, the present invention is effective.
According to the above exemplary embodiment, it is preferable that the light source be a light-emitting diode. By using the light-emitting diode, the size and weight of a device having the surface-emitting devices built therein can be reduced, and manufacturing costs can also be reduced.
According to any one of the above exemplary embodiment, it is preferable that the light guide plate be structured so as not to emit the light traveling in the other direction from its surface before the light reaches the reflecting layer. When the light guide plate is formed so as not to emit light traveling in the other direction of the light introduced from the light source, since all the optical path length of light emitted from the surface of the light guide plate can be made longer than the distance from the light source to the reflecting layer, in-plane uniformity of emitted light can be enhanced.
According to any one of the above exemplary embodiment, it is preferable that the surface of the light guide plate be provided with a convex portion or a concave portion having an inclined face for emitting light from the surface.
According to the above exemplary embodiment, it is preferable that the convex portion or the concave portion be provided with a gently inclined face formed to face the other direction, as viewed from the inside of the light guide plate, and inclined at a small angle to the surface of the light guide plate, and a steeply inclined face formed to face the predetermined direction, as viewed from the inside of the light guide plate, and inclined at a large angle to the surface of the light guide plate. In this case, it is preferable that the convex portion consists of the gently inclined face and the steeply inclined face, have a triangular cross section, and be formed in stripes in plan view. In this case, when the convex portion is formed on the surface opposite from the light emitting direction and the predetermined direction and the other direction are opposite from each other, the steeply inclined face is formed on the light-source side of the convex portion, and the gently inclined face is formed on the side opposite from the light source.
According to any one of the above exemplary embodiment, it is preferable that the light source and the light guide plate be constructed as a front light to be placed in front of the panel surface of a liquid crystal device.
In accordance with another exemplary embodiment, there is provided a front light including a transmissive light guide plate for emitting from its surface light propagating therein in a predetermined direction along the surface, and a light source placed adjacent to an end of the light guide plate so as to introduce light into the light guide plate in the other direction, which is different from the predetermined direction, wherein the light guide plate allows sight therethrough, and a reflecting layer is disposed at an end of the light guide plate different from the end where the light source is placed, so as to reflect light, which propagates inside the light guide plate in the other direction, in nearly the predetermined direction inside the light guide plate.
According to this above exemplary embodiment, light introduced from the light source into the light guide plate propagates inside the light guide plate, is reflected by the reflection layer, propagates in the other direction, and is emitted from the surface of the light guide plate. Since this allows a long optical path length from the light source to the emitting position, even when directivity lies in the light emitting direction of the light source, it can be reduced. This makes it possible to improve in-plane uniformity of illumination light and to reduce nonuniformity of leakage light, thereby reducing deterioration of visibility resulting from directivity of the light source.
According to above exemplary embodiment, it is preferable to place a light-scattering layer before the reflecting surface of the reflecting layer. According to this front light, since light scattered by the light-scattering layer at a distance from the light source is emitted from the light guide plate, operations equivalent to those of the light source having low directivity can be obtained even when directivity lies in the light emitting direction of the light source, and visibility can be further improved. In particular, it is possible to further reduce directivity in a region closer to the light source, as compared with a case in which a light-scattering plate is placed on the incident surface of the light guide plate in the conventional front light structure. This reduction allows a light-scattering plate having a relatively low scattering intensity to be used, which reduces light loss.
According to above exemplary embodiment, the light source may have directivity in the light emitting direction in the light guide plate.
According to above exemplary embodiment, the present invention is effective particularly in a case in which the light source is a point light source, and it is preferable that the light source be a light-emitting diode. According to any one of above exemplary embodiment, it is particularly effective when the light guide plate is constructed so as not to emit light, traveling in the other direction, from the surface before it reaches the reflecting layer.
It may be possible to construct a reflective liquid crystal device in which the above front light is placed in front of a liquid crystal panel. In this case, high visibility can be obtained even in both bright and dark environments.